“As to which, first one must consider the Saviour himself; secondly, His sacraments, by which we obtain salvation; thirdly, concerning the end (_finis_), immortal life, to which we come by rising again through Him. “As to the first, one has to consider the mystery of the Incarnation, in which God was made man for our salvation, and then those things that were done and suffered by our Saviour, that is, God incarnate.” This Prologue indicates sufficiently the order of topics in the _Pars tertia_ of the _Summa_, through Quaestio xc., at which point the hand of the Angelic Doctor was folded to eternal rest. He was then considering _penance_, the fourth in his order of Sacraments. All that he had to say as to the person, and attributes, and acts and passion of Christ had been written; and he had considered the Sacraments of baptism, confirmation, and the eucharist; he was occupied with _poenitentia_; and still other sacraments remained, as well as his final treatment of the matters which lie beyond the grave. So he left his work unfinished, and, in spite of many efforts, unfinishable by any of his pupils or successors.[451] II Inasmuch as the matter of their thoughts was transmitted to the men of the Middle Ages, and was not drawn from their own observation or constructive reasoning, the fundamental intellectual endeavour for mediaeval men was to apprehend and make their own, and re-express. Their intellectual progress followed this process of appropriation, and falls into three stages--learning, organically appropriating, and re-expressing with added elements of thought. Logically, and generally in time, these three stages were successive. Yet, of course, they overlapped, and may be observed progressing simultaneously. Thus, for example, what was known of Aristotle at the beginning of the twelfth century was slight compared with the knowledge of his philosophy that was opened to western Europe in the latter part of the twelfth and the beginning of the thirteenth. And while, by the middle of the twelfth century, the elements of Aristotle’s logic had been thoroughly appropriated, the substantial Aristotelian philosophy had still to be learned and mastered, before it could be reformulated and re-expressed as part of mediaeval thought.